The present invention generally relates to an arrangement for the automatic shifting of an epicyclic speed-change transmission, and more particularly to an arrangement which ensures a smooth, relatively short gear engagement operation.
In a known arrangement of this type, as disclosed by German Patent Specification No. 1 530 584, corresponding to U.S. Pat. No. 3,483,771, the entire disclosure of which is herein incorporated by reference, a two part transmission has a first transmission section, which can be bought into drive connection with the input shaft, and which consists of a linkage, in which two single-carrier epicyclic transmissions are interconnected by two coupling shafts. The second transmission section, which can be brought into drive connection with the output shaft, consists of a single-carrier epicyclic transmission, in which one transmission element of the second transmission section is connected in a torsionally rigid manner to one of the coupling shafts of the first transmission and another transmission element of the second transmission section is connected, by a one-way clutch, to the other coupling shaft of the first transmission section, or in a variant to a non-coupled transmission element of the first transmission section. The one-way clutch engages in first gear and in reverse gear, and then represents a second rigid connection between the two transmission sections in tractive operation. In this known arrangement, although only two shifting means are necessary for forming, in each case, one of the four forward gears and the reverse gear, with only one of the two shifting means changing during a gear shift, there are still also difficulties with the known arrangement during the engagement operation, i.e., when preselecting a gear in the neutral position.
Thus, two things have to be taken into consideration in the engagement operation mentioned.
First, due to the high supporting torques in the forward and reverse gear brakes, the filling time of the fluid actuated shift actuators of the transmission involved in each case during the engagement operation is relatively long.
Second, a cardan shaft torque results from the delay of the turbine impeller mass when using a hydrodynamic torque converter and the proportionate motor masses or remaining transmission masses, in addition to the creeping torque of the torque converter. In other words, this shift actuator filling time period must be correspondingly long in order that the change in torque per unit of time (jerk) does not become too great.
Accordingly, an object of the present invention is to provide an arrangement for the automatic shifting of an epicyclic, shiftable transmission which ensures a short engagement operation with good comfort.
In an embodiment according to the present invention, two transmission sections of a two part transmission operate together, at least in starting gear and, if appropriate, in reverse gear, in the manner of a group gear, because the first and second transmission sections are only interconnected by a single constant torsionally rigid drive connection. As a result, and due to the provision of gradually increasing engagement phases during the preselection of a gear, according to the invention, it is achieved that the mass delay of the drive motor, of the turbine impeller and of the connected transmission and shift elements does not act on the cardan shaft during the engagement operation. Consequently, the engagement operation can only be perceived as a smooth progression from an output torque equal to zero up to an output torque equal to the creeping torque. The engagement operation thus takes place in a considerably shorter time and with good comfort.
In an embodiment according to the present invention, the engagement operation proceeds in such a way that the shift elements for the starting gear, for example, first gear, and for reverse gear are already applied during motor starting in selection position P (parking position) and N (neutral position) of the shift mechanism, but in such a way that the turbine speed of the torque converter is still scarcely reduced. This has the advantage that the drive motor no longer has to inject fuel and thus fuel consumption is not increased. The shift elements of the rear transmission section are either applied only gently or not at all, so that no torque can be transmitted.
When engaging selection position R (reverse gear) or D (complete shift range of all forward gears), the working pressure of pressure-medium actuators of frictional shift elements of the transmission (clutches, brakes) is spontaneously increased, so that the forward transmission section blocks and the turbine speed of the hydrodynamic torque converter is drawn to zero. At the same time, at this moment at the latest, the fluid actuated shift elements of the rear transmission section are applied and, if the turbine speed in the front transmission section becomes zero, then the shift elements of the rear transmission section also fully engage. The shift element of the front transmission section which is not required for the starting gear (for example, a shifting clutch bringing the input shaft into connection in reverse gear with a transmission element of the front transmission section) is released under control, so that the output torque can rise gently to the level of the creeping torque. It is also possible for this process not to take place until the foot brake is released or the accelerator pedal is dabbed.
In a variant embodiment within the scope of the present invention, the engagement operation may also take place in such a way that, as soon as the turbine speed has become zero after the shift elements of the front transmission section have engaged, the unrequired shift element of the front transmission section is abruptly released and the already applied fluid actuated shift element of the rear transmission section is increased gently to the supporting torque of the creeping torque.
The invention can also be used in the same way when the vehicle is in motion. Similarly, the invention may also be used, depending on the cutting in of the shift element of the rear transmission section, as a hill support.